1. Field of the Invention
The present invention relates to a display device which includes an electron-emissive element. Specifically, the invention relates to a display device which includes a transistor in each pixel and a field electron-emissive element for controlling a gray scale by using the transistor.
2. Description of the Related Art
In recent years, a flat panel (flat panel type) display device has been actively developed as an image display device which replaces the mainstream Cathode Ray Tube (CRT). As such a flat panel display device, a display device including electron-emissive elements (also described as field electron-emissive elements) which emit light by electron-beam excitation utilizing electrons emitted by the electric field effect, namely, an electron emission display (FED: Field Emission Display) device has been proposed. An electron emission display device has been attracting attention because of its high display performance of a moving image and low power consumption, and there is an advantage that the contrast of a displayed image is high since it is a display device using self-luminous light-emitting elements unlike a display device using liquid crystals.
FED has a structure where a first substrate having a cathode electrode and a second substrate having an anode electrode provided with a phosphor layer are disposed to be opposed to each other, and the first substrate and the second substrate are sealed with a sealing material. An electron emitted from the cathode electrode moves through space between the first substrate and the second substrate to excite the phosphor layer provided to the anode electrode, so that an image can be displayed by light emission. Both the first substrate and the second substrate are sealed with a sealing material, and the space is kept at a high vacuum.
FED can be classified into a diode-type FED, a triode-type FED, and a tetrode-type FED according to the configuration of electrodes. A diode-type FED has a structure where striped patterned cathode electrodes are formed over a surface of a first electrode while striped patterned anode electrodes are formed over a surface of a second electrode so as to be crossed with the cathode electrode. The distance between the cathode electrode and the anode electrode is several μm to several mm. An electron is emitted from between the cathode electrode and the anode electrode by applying a voltage thereto. A voltage to be applied may be any level of voltage as log as it is less than 10 kV. The emitted electron reaches to the phosphor layer provided to the anode electrode to excite the phosphor layer, so that an image can be displayed by light emission.
A triode-type FED has a structure where an insulating film is formed over a first substrate which is formed with cathode electrodes, extraction gate electrodes are formed to be crossed with the cathode electrodes with the insulating film interposed therebetween. When the cathode electrodes and the extraction gate electrodes are seen from above, they are arranged in stripes or in matrix; and in the insulating film which is in an intersection region of each cathode electrode and each extraction gate electrode, an electron-emissive element which is an electron source is formed. By applying a voltage to the cathode electrode and the extraction gate electrode to apply a high electric field to the electron-emissive element, an electron can be emitted from the electron-emissive element. This electron is pulled toward the anode electrode of the second substrate to which a voltage higher than the voltage of the extraction gate electrode is applied, thereby exciting the phosphor layer provided to the anode electrode, so that an image can be displayed by light emission.
A tetrode-type FED has a structure where a placoid or thin film convergent electrode is formed between an extraction gate electrode and an anode electrode of a triode type-FED, and the convergent electrode has an opening in each pixel. By converging electrons emitted from a light-emissive element in each pixel by such a convergent electrode, the phosphor layer provided to the anode electrode can be exicited, and thus, an image can be displayed by light emission.
As electron-emissive elements, there are a Spindt-type electron-emissive element, a surface-conduction electron-emissive element, an edge-type electron-emissive element, a MIM (Metal-Insulator-Metal) element, a carbon nanotube electron-emissive element, and the like.
A Spindt-type electron-emissive element is an electron-emissive element including a conical electron-emissive element. The Spindt-type electron-emissive element has advantages compared to other electron-emissive elements in that (1) an electron extraction efficiency is high since it has a structure where an electron-emissive element is disposed in a central region of a gate electrode with the largest concentration of the electric field, (2) in-plane uniformity of a current of an electron-emissive element is high since patterns having the arrangement of electron-emissive elements can be accurately drawn to set suitable arrangement for distribution of the electric field, (3) an emission direction of electrons is regulated well, and the like.
As conventional Spindt-type electron-emissive elements, there are a conical electron-emissive element formed by depositing metal (see Reference 1: Japanese Published Patent Application No. 2002-175764), an element formed to have a conical electron-emissive portion using a MOSFET (see Reference 2: Japanese Published Patent Application No. Hei. 11-102637), and the like.
Here, description is made of electrical characteristics of electron-emissive elements with reference to FIGS. 14 and 15. A structure described in FIG. 14 shows an exemplary structure of a light-emitting element in one pixel which uses the passive matrix driving. A structure described in FIG. 14 includes an emitter array where a plurality of electron-emissive elements (hereinafter, also described as emitters) 10 are arranged, an extraction gate electrode 11 for applying an electric field to the emitter array, an insulating film 12 for electrically insulating the extraction gate electrode 11 from the emitter array, an anode electrode 15 provided away from the emitter array with a distance of several μm to several mm, a light-emitting material (also described as a fluorescent material) 16, and a cathode electrode 17.
Note that in this specification, an electrical element having a function of light emission is described as a light-emitting element. That is, an electrical element including the emitter array, the light-emitting material 16, and the anode electrode 15 corresponds to a light-emitting element. Note that the light-emitting element may include the extraction gate electrode 11 as shown in FIG. 14. In addition, the emitter array may be electrically connected to the cathode electrode 17, or the emitter array may be formed over the cathode electrode 17. Further, a potential of the extraction gate electrode 11 is denoted by Veg; a potential of the anode electrode 15 is denoted by Va; and a potential of the cathode electrode 17 is denoted by Vc.
In this specification, connection means electrical connection as long as there is no particular description. On the other hand, separation means a state in which an object is not connected to another object and electrically insulated from another object.
FIG. 15 shows electrical characteristics of the light-emitting element with the structure in FIG. 14 which is in a biased state. FIG. 15 shows a current-voltage characteristic of the light-emitting element in the case of fixing potentials of the cathode electrode 17 and the anode electrode 15 to swing a voltage between the extraction gate electrode 11 and the cathode electrode 17 (Veg−Vc). As shown in FIG. 15, the current-voltage characteristic of the light-emitting element is such that current hardly flows until (Veg−Vc) reaches the threshold voltage of the emitter array (hereinafter, also described as Veth); however, a current flows exponentially and rapidly when (Veg−Vc) becomes higher than Veth. Luminance of the light-emitting element is determined in accordance with the amount of this current, Va which is a potential of the anode electrode 15, Vc which is a potential of the cathode electrode 17, and the characteristics of the light-emitting material 16. For example, if the characteristics of the light-emitting material 16 are the same, and Va which is the potential of the anode electrode 15 and Vc which is the potential of the cathode electrode 17 are the same, luminance of the light-emitting element is dependent on the amount of current flowing to the emitter array. Note that an electric filed of Va which is the potential of the anode electrode 15 mainly works to accelerate electrons emitted from electron-emissive elements, so that it hardly contributes to the current-voltage characteristic of the light-emitting element. That is, a current flowing to the light-emitting element is substantially determined by a voltage between the extraction gate electrode 11 and the cathode electrode 17 (Veg−Vc).
Here, description is made of a driving method of a display device including a light-emitting element. The driving methods of the display device are classified roughly into an active matrix driving method and a passive matrix driving method. A display device using the passive matrix driving can be manufactured at low cost since it has a simple structure where the light-emitting elements are interposed between a matrix of electrodes; however, the passive matrix driving is not always suitable for a large-area or high-definition display device since other pixels cannot be driven while a certain pixel is driven.
In FIG. 14, the emitter array is driven by the extraction gate electrode 11 and the cathode electrode 17 formed in matrix, and a voltage between the extraction gate electrode 11 and the cathode electrode 17 (Veg−Vc) is controlled by applying appropriate potentials to the respective electrodes to control the luminance of the light-emitting element. FIG. 18 shows an example where light-emitting elements driven by the passive matrix driving method are arranged in matrix.
On the other hand, the manufacturing cost of a display device using the active matrix driving method is often higher than a display device using the passive matrix driving since active elements and means for holding luminance information are provided in each pixel; however, even when a certain pixel is driven, other pixels can emit light while at the same time holding luminance information. FIG. 19A shows an example where light-emitting elements driven by the active matrix driving method are arranged in matrix. Although FIG. 19A shows only four light-emitting elements, more than four light-emitting elements are often provided. A display device using an active matrix driving method includes a plurality of data lines 28, a plurality of scan lines 29 which are arranged to be at right angles or about at right angles to the plurality of data lines 28, a plurality of pixel circuits 24 which are arranged in a region where the data lines 28 and the scan lines 29 are crossed with each other, and a plurality of light-emitting elements. The pixel circuits 24 includes a driving transistor Tr1 which is an active element connected to an emitter array in series, a gate electrode potential control circuit 23 of a driving transistor, and a cathode electrode 27. Note that the cathode electrode 27 is an electrode for controlling a potential of one of either a source electrode or a drain electrode of the driving transistor Tr1, and the cathode electrode 27 may be shared with other wires such as the scan lines 29.
FIG. 19B shows an example of the gate electrode potential control circuit 23 of a driving transistor. A transistor 30 is conductive (turned on) when a High signal is input to a terminal S to transmit a potential of the data line 28 connected to a terminal D to a capacitor 31 and a terminal Q (this operation is also described as “data writing”). After that, the transistor 30 is not conductive (turned off) when a Low signal is input to the terminal S not to transmit the potentials of the data lines 28 connected to the terminal D to the capacitor 31 and the terminal Q; therefore, a potential of the terminal Q in the period when the transistor 30 has been on is held in the capacitor 31 until the transistor 30 is turned on again. In accordance with the potentials of the capacitor 31 and the terminal D at this time, Vgs of the driving transistor Tr1 is determined so that a drain current corresponding to Vgs keeps flowing through the driving transistor Tr1. In this manner, the active matrix driving method is realized.
As a conventional electron-emissive display device which uses an active matrix driving method, a display device disclosed in non-patent document 1 (IDW'04 pp. 1225-1228 “HfC coated Si-FEA with a built-in poly-Si TFT”) is given, as an example. In non-patent document 1, an example in which HfC is formed over an emitter which is manufactured with amorphous silicon and sputtering treatment is applied to improve current-voltage characteristics of an emitter array is disclosed. In addition, an example where a thin film transistor (hereinafter, also described as TFT) which is manufactured with polysilicon is connected to the emitter array in series to perform the active matrix driving method is also disclosed.
In a display device using the active matrix driving method which uses a current driving-type light-emitting element, specifically an organic EL element which is an element having two terminals, there are techniques related to a compensating method for luminance variation of light-emitting elements due to the characteristic variation of transistors (see Reference 3: Japanese Published Patent Application No. 2004-246204, Reference 4: Japanese Translation of PCT International Application No. 2002-514320, and Reference 5: Japanese Translation of PCT International Application No. 2002-517806).
In this manner, the compensation for the variation of the transistors in the display device using the active matrix driving method which uses an organic EL element which is an element having two terminals has been examined.
As described above, when light-emitting elements of FED are driven by the active matrix driving method, an active element which controls a current flowing to the light-emitting elements is necessary. A transistor or a thin film transistor can be applied to this active element. In the case of employing a transistor as the active element, a structure as shown in FIG. 16 where an emitter 10 of a light-emitting element of FED and one of either a source electrode or a drain electrode of the driving transistor Tr1 are electrically connected to each other; the other of either the source electrode or the drain electrode of the driving transistor Tr1 is electrically connected to a cathode electrode 27; and a current Ids which flows to the driving transistor Tr1 and the light-emitting element are controlled by controlling a voltage which is applied to the gate electrode of the driving transistor Tr1 (hereinafter, also described as Vgs) can be provided. Note that in a conventional display device, when light-emitting elements of FED are driven by an active matrix driving method, the extraction gate electrode 11 is shared by the whole light-emitting elements and fixed at a certain potential Veg. In addition, the potential of the anode electrode 15 is fixed at Va. At this time, a voltage which is applied between the source electrode and the drain electrode of the driving transistor Tr1 is denoted by Vds, while a voltage which is applied between the extraction gate electrode 11 of the light-emitting elements and the emitter 10 is denoted by Vege.
The current Ids which flows into the driving transistor Tr1 and the light-emitting element, and a potential of the emitter 10 in the case of connecting the light-emitting element and the driving transistor Tr1 to each other as shown in FIG. 16 are described with reference to FIGS. 17A and 17B. In FIG. 17A, a point “a” shows an operating point in the case of applying a high level of voltage (Vgs) between the gate electrode and the source electrode of the driving transistor Tr1 to increase the amount of current Ids which flows into the driving transistor Tr1 and the light-emitting element in order to increase the luminance of the light-emitting element; a solid line A shows the current-voltage characteristics of the driving transistor Tr1; and a solid line B shows current-voltage characteristics of the light-emitting element. On the other hand, in FIG. 17B, a point “a” shows an operating point in the case of applying a low level of voltage Vgs between the gate electrode and the source electrode of the driving transistor Tr1 to decrease the amount of current Ids which flows to the driving transistor Tr1 and the light-emitting element to decrease the luminance of the light-emitting element; a solid line A shows the current-voltage characteristics of the driving transistor Tr1; and a solid line B shows the current-voltage characteristics of the light-emitting element.
The source-drain voltage Vds of the driving transistor Tr1 is relatively low when the luminance of the light-emitting element is high as shown in FIG. 17A, while the source-drain voltage Vds of the driving transistor Tr1 is decreased in order to decrease the luminance of the light-emitting element. From FIGS. 17A and 17B, the scope of Vds can be represented by the following formula 1.0<Vds<Veg−Vc−Veth  [formula 1]
Here, by quoting a voltage value disclosed in non-patent document 1, (Veg−Vc) is about 5 V and Veth is about 35 V. That is, a maximum value of Vds can be estimated to be about 20 V from the formula 1.
In this manner, when the light-emitting element of FED is driven by an active matrix driving method, a very high voltage is applied to the driving transistor Tr1 differently from the case of using an organic EL element. This point is one of the problems in the case of driving electric field electron-emissive light-emitting elements using the active matrix driving method. Thus, a pixel circuit of a display device which is driven by the active matrix driving method using the organic EL element cannot be simply employed since a very high voltage is applied to a transistor. In non-patent document 1, in order to make the driving transistor Tr1 endure this high voltage of 20 V, measures such as lengthening a channel length of the driving transistor Tr1, and making the gate electrode of the driving transistor Tr1 into a tine shape are taken.
However, even if efforts to increase the withstand voltage of the driving transistor Tr1 are made, the driving transistor Tr1 is easily deteriorated when a high voltage is continuously applied thereto. In addition, when a high voltage is continuously applied to the transistor, the reliability thereof is extremely decreased. This makes the yield of products decrease, so that it is very disadvantageous in cost as well. Accordingly, a voltage which is applied to the transistor is desirably as low as possible.
In addition, for an active matrix display device using a light-emitting element such as an organic EL element, there are techniques related to a compensating method for luminance variation of the light-emitting elements due to the characteristic variation of transistors as shown in Reference 3 to Reference 5. In an electric filed electron-emissive display device using the active matrix method which uses an electron-emissive element, a compensating method for the luminance variation of light-emitting elements due to the characteristic variation of transistors, the variation of the light-emitting elements, characteristic deterioration of the light-emitting elements, or the like becomes important.